NEW DELHI, March 5 (Xinhua) -- Indian investigators have arrested four senior executives of companies owned by disgraced billionaire jeweller Nirav Modi and his uncle Mehul Choksi in connection with the the country's biggest-ever bank fraud of 1.8 billion U.S. dollars.

"The four executives were arrested Sunday. They included a director of Choksi's company Gili India and a top auditor. All were involved in preparing applications for fraudulent Letters of Undertakings submitted to state-owned Punjab National Bank," an official said Monday.

With these arrests, the number of those taken into custody in connection with the fraud has risen to 17. Though Nirav Modi has not yet been arrested, India's External Affairs Ministry has revoked his passport as well as Choksi's over their involvement in the scam.

Indian authorities have also launched a massive crackdown on companies linked to Nirav Modi, and seized 10,000 luxury watches and nine luxury cars last week, apart from freezing his personal shares and mutual funds.

Nirav Modi is said to have defrauded Punjab National Bank, India's second largest state-run bank, of 1.8 billion U.S. dollars, though he has said that he owed the bank only 775 million U.S. dollars, in a letter shot to the bank's management.

The celebrity jeweller, the alleged mastermind of the massive fraud, is said to have fled the country and was reportedly last seen in New York after his appearance at World Economic Forum in Davos as a member of the Indian delegation.